The present invention relates to a bus driver circuit for a two-conductor bus system connected to a control unit containing the bus driver circuit and multiple actuator modules and/or sensor modules in a motor vehicle, with the bus driver circuit supplying signals in the form of a pulse train formed by different voltage levels into the healthy bus conductor in the event of a short circuit on one of the two bus conductors.
Actuators and sensors are being installed in increasing numbers in motor vehicles to regulate the power train, brake systems, driving performance and restraint systems for the protection of occupants of the vehicle. Heavy and bulky cable harnesses can be eliminated by introducing bus systems linking together the actuators, the sensors and the respective control units.
In the case of a bus for a safety-relevant system such as a restraint system in particular, measures must be taken so that a fault on the bus conductors will not result in a system failure resulting in the restraint systems not being deployed in the event of a crash situation. For such a bus, there is therefore the requirement that signal transmission must be possible between a central control unit and actuators and/or sensors connected to the bus even in the event of a short circuit to the battery voltage of the vehicle or to ground on one of the two bus conductors. For this reason, the German Published Patent Application No. 198 13 952 describes a bus driver circuit with which it is possible to transmit messages in the form of pulse trains formed by two different voltage levels over the intact bus conductor in the event of a short circuit on one of the two bus conductors. The bus driver circuit is a series connection of three switches at two different voltage potentials.
An object of the present invention is to provide a bus driver circuit which requires the least possible circuitry complexity and permits data to be injected into the bus in such a form that it can be identified reliably by the actuator modules and/or sensor modules connected to the bus.
This object is achieved through the fact that the bus driver circuit is composed of multiple electrically controllable switches forming an H-bridge configuration, at least one switch being present in each of the five H-bridge branches, the two bus conductors are each connected to one end of the shunt arm of the H-bridge, and the individual switches in the longitudinal branches of the H-bridge can be switched into at least two positions which are at different voltage levels.
Accordingly, it is expedient that the switches in two longitudinal branches of the H-bridge, starting from different ends of the shunt arm, can be switched to positions which are at different high voltage levels, and the switches in the two other longitudinal branches can be switched to positions which are at different low voltage levels. In the event of a short circuit on one of the two bus conductors, the switches in the longitudinal branches of the H-bridge that are connected to the healthy bus conductor are switched to switch positions so that a desired pulse train occurs on the healthy bus conductor. In the event of a short circuit in a bus conductor to ground, a switch connected to this bus conductor and having a switch position at ground potential is preferably switched to this position.
In the case of messages having a high transmission rate, the at least one switch in the shunt arm is expediently closed briefly between switching through different voltage levels over switches in the longitudinal branches of the H-bridge. This increases steepness of the edges of the pulses transmitted, thus permitting a high pulse transmission rate. Messages requiring a high transmission rate include, for example, deployment commands for restraint systems.
The pulse trains transmitted are preferably Manchester encoded, thus permitting easy synchronization of actuator modules and/or sensor modules.